For editing, I think snapseed is good. The interface is quirky, but good once you get used to it.
For actually taking photos on the iPad, camera boost. And yes, I wrote that app so I might be biased, but it was designed right from the start to make the iPad cameras a bit more useful. It takes full-sized photos (1280x720 instead of the camera app's 960x720), it has a night mode that means your photos aren't buried in grain, there's realtime noise reduction too, and also a very high end image processing system that lets you do things like colour adjustment, light + contrast etc. and effects in realtime. All of that works with HD video too with the exception of the night mode.